1. Field of the Invention
The present invention relates to a displacement detecting device adapted to detect the displacement in a direction perpendicular to a surface having a diffraction grating arranged thereon by using interference of light.
2. Description of the Related Art
Conventionally, displacement detecting devices for detecting the displacement in a direction perpendicular to a surface-to-be-measured are widely used. An example of such displacement detecting devices is described in Japanese Unexamined Patent Application Publication No. H06-241728. In the displacement detecting device described in Japanese Unexamined Patent Application Publication No. H06-241728, two pairs of monochromatic lights are used, each pair including two monochromatic lights having slightly mutually different frequencies. The two light-beams of each pair are incident on a diffraction grating of a first object and a diffraction grating of a second object. The two light-beams have mutually different incidence angles on both sides of the optical axis, and the incident directions of the two light-beams are asymmetrical to each other with respect to the optical axis. The two pairs are reverse-symmetrical to each other on both sides of the optical axis.
Next, two or more beams of diffracted lights, which have been subjected to light heterodyne interference, are taken out from two positions symmetrical to each other on both sides of the optical axis. Thus, phases φxz, φxz′ of light heterodyne detection signals are detected based on variation of optical path difference generated due to a position deviation in an X-direction parallel to a surface having two diffraction gratings arranged thereon and a position deviation in a Z-direction perpendicular to the surface having two diffraction gratings arranged thereon. Further, two-dimensional phase changes φx, φz of the two objects in the X-direction and the Z-direction are calculated based on sum and difference of the expression of phase φxz and the expression of phase φxz′.